1. Field of the Invention
This invention relates to the field of communications, and in particular to a system and method for the initial acquisition/determination of the phase and frequency of a pilot CDMA signal.
2. Description of Related Art
CDMA (Code-Division Multiple Access) systems are common in the art, particularly in the field of mobile communications. Multiple communication channels are provided within the same frequency band by ‘spreading’ the information content of each channel across the frequency band by modulating the information using a specific code pattern. The code pattern is designed such that the modulated signal appears as noise to any demodulator that is not in phase with the modulated signal. That is, a demodulator that applies the same code pattern to the modulated signal coincident with the application of the code pattern to the information signal will produce a reproduction of the information signal. On the other hand, a demodulator that applies either a different code pattern, or the same code pattern at a different phase, than the modulation system, will produce an incoherent signal. The different code patterns that provide this desired coherent/incoherent effect are referred to as “orthogonal” codes. Of particular note, each of the incoherent signals will exhibit the same characteristics as random noise, and common noise-filtering techniques can be employed to minimize their adverse effects.
In a multiple communication CDMA system, therefore, when a demodulator is ‘tuned’ to, or ‘in-phase’ with, a particular CMDA transmitter, the demodulated signal will appear as a coherent signal within a bed of noise that is produced by all of the other ‘out-of-phase’ CDMA transmitters.
For a demodulator to be in-phase with a transmitted signal, the start of the demodulating code must occur within the transmitted stream at the same point that the modulating code was applied to the information signal. To remain in-phase, the demodulating code must be applied to the transmitted stream at the same frequency that the modulating code was applied to the information signal. Thus, proper tracking of a transmitted signal requires a determination of both the frequency and the phase of the transmission. This determination of frequency and phase of a transmission is defined as the acquisition of the transmission.
To facilitate acquisition when communication is initiated between a transmitter and receiver, the transmitter transmits a pilot-signal that includes a CDMA modulation at a particular phase. The receiver applies the CDMA code to the received transmission at progressively different phases until a coherent signal is detected. When the coherent signal is detected, the receiver locks onto the transmission frequency, using conventional AFC (Automatic Frequency Control) techniques. To detect the coherent signal, the receiver must be operated at approximately the same frequency as the transmitter. Otherwise, even if the demodulation code starts at the same point in the stream that the modulation code started (i.e. is in-phase with the modulation code at the start of the code sequence), a substantial difference between the transmitter and receiver frequency will cause the demodulation code to be out of phase with the modulation code before the end of the code sequence.
In a conventional CDMA system, the receiver is operated at a variety of frequencies about the nominal transmission frequency, and at each frequency, the aforementioned phase-variation-until-coherency process is performed until a coherent signal is detected. When a coherent signal is detected, indicating that the receiver is operating at the frequency and phase of the transmitter, the receiver ceases both the frequency and phase searching, and activates an AFC process to maintain frequency-lock with the transmitter.
Once the transmission signal is acquired, the receiver terminates the search-mode and enters a steady-state mode for providing communications between the transmitter and the receiver, using the acquired frequency and phase to effect proper demodulation. U.S. Pat. No. 6,134,260 “METHOD AND APPARATUS FOR FREQUENCY ACQUISITION AND TRACKING FOR DS-SS CDMA RECEIVERS”, issued 17 Oct. 2000 to Bottomley et al, discloses techniques for maintaining frequency and phase tracking after the initial frequency and phase are determined, and is incorporated by reference herein as typifying a conventional prior-art CDMA system.
Because the frequency and phase information is assumed to be correct when the receiver enters the steady-state mode, and the transmitter is assumed to be stable, and because rapid variations in receiver frequency or phase can introduce distortions on the demodulated signals, the feedback loop that is employed for continued tracking in the steady-state mode generally has a long response time and/or a heavily dampened response. Although this dampened response is preferable once the receiver has properly acquired the transmitter, the dampened response has the effect of retaining an improperly acquired transmitter phase. Specifically, if the initially determined phase is incorrect, the dampened response will introduce a substantial delay before the proper phase is determined. While an incorrect phase is being used, the likelihood of demodulation errors is significantly higher than a demodulation at the correct phase.